Primary and Secondary Standards

Standard substance solutions are very important in Analytical chemistry to calibrate instruments, verify accuracy of analytical methods, and quantify (measurement of concentration) unknown analytes with high precision.

Primary Standards

Those substances which can easily be obtained in highly pure and crystalline form and used in preparation of standard solution are known as Primary Standard Substances.

Primary Standard solutions act as known references, because the accurately weighed quantities of these primary standards are used to prepare primary standard solutions, which are used in the standardization of solutions of unknown strength.

Properties of Primary Standards

Primary Standards are extremely pure substances:

A primary standard material should be extremely pure which means that it should be a chemical of high grade of purity, preferably 99.98%/p. High purity ensures that the measured mass corresponds exactly to the chemical species of interest.

Stability on Storage:

A primary standard must be chemically stable over time and under normal laboratory conditions
. It should not decompose, oxidize, reduce, or react with moisture, oxygen, carbon dioxide, or light.

High Molecular Weight:

A suitable primary standard should possess a high molecular weight (or equivalent weight) to minimize errors associated with weighing and solution preparation.
every measurement carries a small instrumental or handling error, particularly during weighing. When the substance has a high molecular weight, the relative effect of this error becomes smaller, thereby increasing the accuracy of the standard solution’s concentration.

Known and definite chemical composition:

The chemical formula of the substance must be well-defined and constant.
It should not exist in multiple hydrated or variable forms that could alter its molecular weight.


MgSO₄ has a molecular weight of 120 and
MgSO₄.7H₂O has molecular weight of 246.

So 120 gram Anhydrous MgSO₄ will give 120 gram of MgSO4,
But 120 gram of MgSO₄.7H₂O will give approx. 58 gram of MgSO4
because MgSO₄.7H₂O has weight of anhydrous MgSO₄ plus extra weight of 7H₂O molecules.
So the actual weight of MgSO₄ is 58 gm, rest of the weight is of water molecules.
So by mistake if we take 120 gm of MgSO₄.7H₂O instead of MgSO₄, then the actual concentration of MgSO₄ become 58 gm, instead of 120 gm in the solution and we get incorrect results.
So selection of chemical as primary standard is very important.

Examples of Primary Standards

For Acid–Base titration: Sodium carbonate, Potassium hydrogen phthalate (KHP), Oxalic acid dihydrate
For Redox titration: Potassium dichromate, Potassium bromate, Potassium iodate
For Precipitation titration: Silver nitrate, Sodium chloride

Secondary Standards

Unlike primary standards, the concentration of solutions of Secondary standard substances, changes with time, thus they need to be standardized frequently before the use.

a secondary standard is a reagent whose concentration cannot be accurately determined by direct preparation and must therefore be standardized against a primary standard.

This is because such reagents do not meet the criteria of a primary standard — mainly due to instability, impurities, or reactivity with air or moisture. As a result, their actual concentration may change over time or differ from the theoretical value calculated from the weighed mass.

The reason may be:

Lack of stability:

Example: NaOH reacts with atmospheric CO₂ to form Na₂CO₃, reducing its actual concentration and base strength.

Hygroscopic nature:

NaOH pellets absorb both moisture and CO₂, hence, their actual composition is not fixed or not predictable during weighing to make accurate preparation.

Impurities:

Impurities that affect the actual mass of substance and create variations in concentration.

Instability:

KMnO₄ decomposes slowly, forming MnO₂ and oxygen gas.
Due to this the actual concentration of KMnO₄ gradually decreases with time and varies. That’s why it is essential to calculate the exact solution concentration at the time of use.